This invention relates to capacitive sensors for sensing the presence of material, and particularly to a capacitive fluid level sensor having switched capacitors and modular circuitry.
As discussed by Garwood in "Noncontact Presence Sensing: What Works Where and Why", Sensors, August 1988, pages 19-21, capacitive sensors are often preferred for fill level sensing and nonmetallic object sensing. Capacitors have the particular advantage of being directly immersible into solids and liquids. They are, however, subject to changes in dielectric constant. The very advantage that creates their unique applicability also reduces their accuracy due to changes in temperature and humidity, or material makeup.
In order to compensate for these environmental and subject material changes, capacitive circuits have been constructed to sense the material independent of dielectric changes in the material and environment. For instance, in U.S. Pat. No. 4,418,567, Kuhnel discloses a capacitive sensor containing a reference "full" capacitor fully submersed in a fluid to be measured, for compensating for changes in the dielectric of the fluid. Another "empty" capacitor, always excluded from the fluid, compensates for the dielectric effect of air. A level-sensing or "measurement" capacitor is partially submerged to sense the fill level of the fluid as represented by the combined dielectric effect of fluid and air between the capacitor electrodes.
Kuhnel applies an amplitude-controlled alternating voltage to each of the capacitors. The resulting signal from the capacitors is multiplied by a 90.degree. phase-shifted alternating voltage signal to produce a voltage level representative of the capacitance of each of the capacitors. The measurement capacitance voltage and full capacitance voltage are each applied to the noninverting inputs of respective differential amplifiers. The empty capacitance voltage is input to the inverting inputs of both of these differential amplifiers.
The output of the amplifier with the measurement capacitance voltage input is an output voltage proportional to the level of filling. The output of the other amplifier is input into the inverting input of a third differential amplifier for feedback to yet another amplifier controlling the amplitude of the alternating voltage applied to the three capacitors.
This design is simplified somewhat in a circuit disclosed by Berryman et al. in U.S. Pat. No. 4,467,646. In addition to a disclosure of specific electrode structure, Berryman et al. describe a circuit in which "full" compensation and measurement capacitors are connected directly to inverting inputs of corresponding amplifiers as well as to amplifier-feedback circuits connected to the amplifier outputs.
The noninverting inputs of the amplifiers and an empty-position compensation circuit receive an amplitude-controlled oscillator output. The full and measurement amplifier and the empty-position compensation circuit outputs are treated the same as the voltage level outputs in the circuit disclosed by Kuhnel.
These circuits are generally effective for providing filling level sensing. However, they still require multiple stages of capacitor signal amplification and manipulation. Further, the capacitor charging is provided by varying the applied voltage.